feat(core): implement physical types with NewType pattern

Story 1.2: Physical Types (NewType Pattern)

- Add Pressure, Temperature, Enthalpy, MassFlow types
- Implement SI base units with conversion methods
- Add arithmetic operations (Add, Sub, Mul, Div)
- Add Display and Debug traits
- Comprehensive unit tests (37 tests)
- Add PSI and Fahrenheit conversions
- Code review fixes applied

All tests passing, clippy clean

crates/core/Cargo.toml

@@ -0,0 +1,15 @@
+[package]
+name = "entropyk-core"
+version.workspace = true
+authors.workspace = true
+edition.workspace = true
+license.workspace = true
+repository.workspace = true
+description = "Core types and primitives for Entropyk thermodynamic simulation library"
+
+[dependencies]
+thiserror.workspace = true
+serde.workspace = true
+
+[dev-dependencies]
+approx = "0.5"